Well, a number of things might be suggested, most obviously that time travel into the past is unlikely because of the number of paradoxes it would create as well as the lack of time travelers we've seen. Some have argued that a Multiverse composed of branching, parallel universes would make reverse time travel possible because, in such a system, universes and events would be more plastic and alterable. Time travel to the past would open up a new, nearly congruent universe, though with some differences. You may also be interested in reading about the grandfather paradox. Time travel to the future, meanwhile, is something allowed by the theory of relativity, and goes into effect as one entity moves closer to the speed of light relative to another object, resulting in the two experiencing time passing at different rates. When the "faster" object slows, it finds that less time has passed for its than for its immobile counterpart. Back to Wikipedia for more about the basis of forward time travel. This isn't my field, though.